1. Field of the Invention
The present invention relates to an image forming apparatus which prints an image as a result of forming a latent image on a recording drum and developing the latent image using toner.
Recently, electrophotographic printing apparatuses (printers) are becoming widespread for printing an image on a recording paper sheet. In such apparatuses, a latent image is formed on a recording drum, the latent image is then developed using toner to obtain a toner image, the toner image is transferred to a recording paper sheet, and the thus-transferred image is then fixed on the paper sheet. Responding to a demand to miniaturize the apparatuses, serial-type apparatuses are being proposed which apparatuses print a relevant image one line at a time, each line being a line extending along a width direction of a recording paper sheet. Apparatuses of this type for personal use having further miniaturization and cost reduction are demanded.
2. Related Art
Serial-type printers include wire dot printers, thermal printers, and electrophotographic printers.
In each of the wire dot printers and thermal printers, a print head is mounted on a carriage which moves along a direction perpendicular to a recording paper sheet conveying direction. The print head approaches a carried recording paper sheet, and then, performs a printing operation as the carriage moves.
In such a printer, a print head retreating mechanism is provided for preventing the print head and/or a recording paper sheet from jamming a paper sheet passing space. A generally known structure of the print head retreating mechanism includes a motor or a solenoid to move a guide shaft guiding movement of the carriage. As a result of the guide shaft being moved by the motor or solenoid, the print head retreats from the paper sheet passing space.
Alternatively, a print part retreating mechanism for print part retreating is applied to electrophotographic page printers. This print part retreating mechanism causes a transfer roller and a fixing roller facing an image carrying body (recording drum) to retreat after a printing operation has been finished. Thus, the image carrying body is prevented from being adversely affected (being corroded by contaminating components) from a transfer roller. Further, incomplete printing and afterimage phenomena are prevented. Further, the fixing roller retreating mainly prevents a recording paper sheet from jamming the paper sheet passing space.
With reference to FIGS. 1A and 1B, a serial-type electrophotographic printer in the related art will now be described. FIG. 1A shows a plan view and FIG. 1B shows a front elevational A--A (in FIG. 1A) sectional view. A reference numeral LE shown in the accompanying drawings including FIG. 1B shows the left edge of a recording paper sheet. In the serial-type electrophotographic printer 11.sub.A, a carriage 12 consists of a process unit 13 and fixing unit 14, is guided by a shaft 15, and is driven by a driving motor 16 through a belt 16a. Thus, the carriage 12 moves above a transfer unit (print platen) 17 in a main scan direction perpendicular to a recording paper sheet conveying direction. A conveying roller 18 is provided in the rear of the carriage 12 as shown in FIG. 1A, and another conveying roller (not shown in the figures) is provided in front of the carriage 12. These conveying rollers convey a recording paper sheet 19 between the transfer unit 17 and the carriage 12 as shown in FIG. 1B.
In the process unit 13, an image carrying body 21 is rotated at a rotation speed such that a circumferential speed of the body 21 is the same as a movement speed of the carriage 12. A surface of the body 21 is uniformly charged by a charger 22 and a exposure unit 23 forms an electrostatic latent image on the surface of the body 21. The electrostatic latent image is developed to be a visible toner image as a result of toner 25 being supplied by a developer 24. The toner image thus formed on the body 21 is transferred to the recording paper sheet 19 by the transfer unit 17 which faces the body 21 via the recording paper sheet 19. After that, toner remaining on the body 21 is removed by a cleaner 27.
The thus-cleaned surface of the body 21 is again charged by the charger 22 and then a similar printing process is repeated. As the printing process is being thus repeated, the carriage 12 moves from the left end to the right end (in the main scan direction) of the recording paper sheet 19 as shown in FIG. 1B. Thus, a predetermined length of image part is printed. After that, the recording paper sheet 19 is forward moved a predetermined length (in the recording paper sheet conveying direction, from the bottom to the top in FIG. 1A) and the carriage 12 is moved from the right side to the left side in FIG. 1B returned to a predetermined position (a home position). Then, the carriage 12 is again moved in the main scan direction and then a similar printing operation is repeated.
The image transfer operation performed through the transfer unit 17 is performed in a state in which a predetermined electric potential is applied between the transfer unit 17 and the image carrying body 21.
The fixing unit 14 is provided with a fixing roller 28 and a heater 29 provided in proximity to the fixing roller 28. A pressure is applied to the recording paper sheet 19 by the fixing roller 28. The fixing unit 14 also consists of a silicon oil coating unit 30 provided adjacent to the fixing roller 28 which supplies silicon oil to the fixing roller 28 so that toner is prevented from adhering to the fixing roller 28. The fixing roller 28 is previously heated to a predetermined temperature before the process unit 13 performs the printing operation. A temperature of the fixing roller 28 is detected by a temperature detector such as a thermistor and is controlled while the process unit 13 is performing the printing operation. The fixing unit 14 moves together with the process unit 13 and fixes an image part which has been printed by the process unit 13.
With regard to FIGS. 2A, 2B and 2C, another sort of serial-type electrophotographic printer will now be described. FIG. 2A shows a plan view, FIG. 2B shows a front elevational A--A (in FIG. 2A) sectional view, and FIG. 2C shows a left-side elevational B--B (in FIG. 2A) sectional view. The serial-type electrophotographic printer 11.sub.B has a structure similar to that of the printer 11.sub.A shown in FIGS. 1A and 1B. However, the carriage 12 is guided by shafts 15a and 15b, and each of the image carrying body 21 and fixing roller 28 has a respective one of pinion gears 21a and 28b provided at one end of a rotation shaft thereof as shown in FIG. 2B. Further a rack gear 31 is provided which extends along the main scan direction in which the carriage 21 moves. Each of the pinion gears 21a and 28b engages with the rack gear 31.
The engagement between each of the pinion gears 21a and 28b and the rack gear 31 causes each of the image carrying body 21 and fixing roller 28 to be rotated as the carriage 12 moves in the main scan direction. Rotation speeds of the rotation of the image carrying body 21 and fixing roller 28 are ones such that respective circumferential speeds thereof are the same as a movement speed of the carriage 12.
Other than the above-described printers in the related art, another sort of serial-type electrophotographic printer is known. In this printer, no unit such as the fixing unit 14 is provided. Instead, a fixing unit having a length longer than a width of a recording paper sheet is provided in front (in the recording paper sheet conveying direction) of the carriage 12. A serial printer having the above-mentioned structure and also having a further structure is also known. The further structure is one, for example, in which a differential mechanism is inserted between the carriage and image carrying body, and a wire and pulleys are used in driving the carriage and image carrying body. In the further structure, rotation of the image carrying body is started when the carriage is located at the home position, and movement of the carriage is started when the image carrying body is rotated a predetermined rotation angle.
Structures of serial-type electrophotographic printers such as those described above are effective to miniaturize the printers. However, if a mechanism such as the above-described print head retreating mechanism or the above-described print part retreating mechanism is provided in such structures of the serial-type electrophotographic printer for the same purpose, the outward dimensions of the printer are enlarged and thus the costs increase.
Further, if a structure of a serial-type electrophotographic printer such as that in which only the process unit is provided in the carriage and the wire and pulleys are used to rotate the image carrying body before the actual printing operation starts is used, other problems may occur. That is, slipping may occur between the wire and pulleys so that it is difficult to accurately control the rotation angle of the image carrying body. If a rack gear and pinion gears are used instead of the wire and pulleys, this problem may be eliminated. However, in the case using the rack and pinion gears, an addition of an extra distance to the rack gear is necessary in order to rotate the image carrying body before the carriage reaches a printing starting position. Thus, the outward dimensions of the printer are enlarged.
Further, the structures of the printers 11.sub.A and 11.sub.B shown in FIGS. 1A, 1B, 2A, 2B and 2C are effective to miniaturize the outward dimensions of the printers because the fixing unit is contained in the carriage. However, it is necessary that each of the circumferential speeds of the body 21 and fixing unit 28 is the same as the moving speed of the carriage 21 on the recording paper sheet 19 while the printing operation is being performed on the recording paper sheet 19. Otherwise, an image to be printed on the recording paper sheet 19 may be degraded. However, in the structure shown in FIGS. 1A and 1B, due to a variation in friction resistance between the recording paper sheet and each of the body 21 and fixing unit 28, it is difficult to cause each of the circumferential speeds of the body 21 and fixing unit 28 to be the same as the moving speed of the carriage 21 on the recording paper sheet 19. The structure shown in FIGS. 2A, 2B and 2C may eliminate this problem. However, in the case using the rack and pinion gears, as mentioned above, the addition of an extra distance to the rack gear is necessary in order to rotate the image carrying body before the carriage reaches a printing starting position. Thus, the outward dimensions of the printer are enlarged.
Further, in each of structures such as those shown in FIGS. 1A, 1B, 2A, 2B and 2C, another problem may occur. That is, if unfixed toner is unexpectedly transferred to a part, other than the recording paper sheet, such as the transfer unit in the printer, a rear side of a subsequently supplied recording paper sheet may be stained. Further, if the thus-transferred toner is then transferred to the fixing roller, the thus-transferred toner is then unexpectedly transferred to the recording paper sheet in the subsequent printing operation.